There is a growing interest towards development of renewable fuels as a result of increasing global energy consumption, finite petroleum resources, and global warming concerns. Non-food biomass materials, such as microalgae, can be viable feedstocks for environmentally sustainable biofuels. In general, microalgae have greater areal productivity than terrestrial plants, can be grown on non-agricultural and marginal lands, and can use low quality water and nutrients from waste streams. Several strains of microalgae are known to accumulate triglycerides—a platform chemical that is in current use for the production of biodiesel as well as high value oleochemicals. However, a key bottleneck in the commercial development of algal bio-refineries is a lack of scalable and viable conversion processes that can produce fuels as well as value-added chemicals.
One approach to downstream processing involves extraction of triglycerides from algal cells using organic solvents such as chloroform and hexane. However, due to the microscopic cell size and robust cell walls, this approach requires additional mechanical disruption. After extraction, the solvent must be separated, usually through evaporation, to recover the triglycerides. The recovered triglycerides may then be further converted to hydrocarbon fuels via thermo-catalytic de-carboxylation or hydro-cracking. Alternately, if biodiesel is the desired product, fatty acid methyl esters (FAMEs) may be more easily obtained from cellular triglycerides through in situ transesterification where oleaginous biomass is directly reacted with a mixture of methanol and catalyst without prior solvent extraction. However, FAME recovery from the reaction mixture still requires solvent extraction (e.g. with chloroform or hexane) followed by solvent evaporation. Solvent extraction methods have, so far, proven effective only with dry biomass, and in situ transesterification is likely more sensitive to even small amounts of moisture in the biomass. In methods involving solvent use, the post-extraction solid residues, generally rich in protein, may also need extensive treatment for solvent removal before use as animal feed or fertilizer.
As an alternative to solvent extraction, thermochemical conversion processes such as pyrolysis and hydrothermal liquefaction can be employed to obtain bio-oil or bio-crude for subsequent conversion to liquid fuels and value-added chemicals. Thermo-chemical methods are generally less species-sensitive than solvent extraction. In addition, these processes can produce fuel/chemical precursors from even the non-triglyceride portions of algal cells (e.g. carbohydrates, other lipids, and proteins). However, thermo-chemical processes, as traditionally applied, produce bio-oils/bio-crude that contains a complex and highly heterogeneous mixture of chemical compounds—long chain fatty acids from degradation of triglycerides and other cellular lipids, short chain oxygenates (e.g. aldehydes, ketones, organic acids, water, and alcohols) from degradation of carbohydrates and N-compounds from protein degradation. Oxygenates in bio-oil lower its heating value and degrade/polymerize over time to produce humins or char. In addition, algal bio-oil/bio-crude obtained from traditional thermochemical processes would consist of a broad molecular weight distribution of chemical species—longer chain products from triglyceride degradation and lower molecular weight compounds from degradation of carbohydrate and protein—that would necessitate further distillation into suitable fuel fractions and result in additional energy inputs for fuel production.